Electrochemical Gas Sensor

ABSTRACT

A gas detector includes an electrochemical gas sensor. The sensor includes a plurality of electrodes. At least one of the electrodes is formed of a catalyst/binder slurry which is halftone printed onto a substrate. The composite printed element and substrate are sintered to form the electrode.

FIELD

The application pertains to electrochemical gas sensors and gasdetectors which incorporate such sensors. More particularly, theapplication pertains to such sensors which can be formed in part byprinting.

BACKGROUND

Electrochemical gas sensors are well known for detecting and quantifyingtoxic gases such as carbon monoxide, oxygen and the like. Such sensorscan be implemented using electrochemical cells. Such cells operate in anamperometric mode providing a current output which is related to theconcentration of the particular analyte gas.

Such sensors usually include a sensing electrode. Known electrodes aremade by a solution based method.

In such solution-based methods, a catalyst is initially ultrasonicallydispersed in an aqueous solution to form a suspension.Polytetrafluoroethylene (PTFE) is added to the suspension for form aflocculate mixture. The flocculate mixture is then transferred onto asubstrate, which is sintered at an elevated temperature. The sinteredmixture is then transferred onto a microporous PTFE membrane, thenpressed. The ratio of PTFE in the electrode not only affects gasdiffusion parameters in the sensor, it also supports the electrocatalystand maximizes the interfaces between catalyst, gas and electrolyte atwhich the key electrochemical processes occur.

As is apparent, many steps are needed in this solution-based method tomanufacture an electrode. The consequences include high manufacturingcosts, material costs and labor costs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a gas detector in accordance herewith;

FIG. 2 is a flow chart illustrating aspects of a method in accordanceherewith;

FIG. 3 is a graph illustrating response time to O₂ in air for anelectrode including a mixture of GEFC-IES and platinum;

FIG. 4 is a graph illustrating response time to CO in air for anelectrode including a mixture of GEFC-IES and platinum; and

FIG. 5 is a graph illustrating response time to O₂ in air for anelectrode including a mixture of GEFC-IES, platinum and graphite.

DETAILED DESCRIPTION

While disclosed embodiments can take many different forms, specificembodiments hereof are shown in the drawings and will be describedherein in detail with the understanding that the present disclosure isto be considered as an exemplification of the principles hereof, as wellas the best mode of practicing same, and is not intended to limit theclaims hereof to the specific embodiment illustrated.

In one aspect, an electrochemical gas sensors having improvedproductivity can advantageously be implemented by using screen printingtechnology. A catalyst slurry, or recipe, can be screen printed orhalftone printed on an electrode membrane by a printer, then sintered.

The printed element can then be used as an electrode of anelectrochemical sensor. Exemplary types of sensors include O₂ sensors orCO sensors. In another aspect, alternative types of sensors inaccordance herewith include, without limitation, oxygen pumps and toxicgas sensors.

The slurry can be made simply and quickly without any need forcomplicated equipment. The slurry can include a catalyst, binder, anddiluents. Unlike known processes, the screen printing method, inaccordance herewith, has fewer steps.

The catalyst can be platinum, platinum black, a mixture of graphite andplatinum, a mixture of carbon and platinum black, a noble metal,mixtures thereof.

A solution of perfluorinated ion electrolyte solution (GEFC-IES thecopolymer of perfluorosulfonic acid and PTFE) commercially availablefrom Golden Energy Fuel Cell Co., Ltd. or Nafion® (copolymer oftetrafluoroethylene (Teflon®) andperfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid) commerciallyavailable from Dupont™, can be used as a binder. Glycol or other similarchemicals can be used as a diluent to form a catalyst slurry, recipe orcatalyst system, which can be printed on a PTFE membrane by a printer.The printed element is sintered at an elevated temperature to form anelectrode which can be used in an electrochemical sensor such as O₂sensor or CO sensor.

GEFC-IES's or Nafion®'s function is that of a binder. Its ratio in theelectrode not only affects gas diffusion parameters in the sensor whilstsupporting the electrocatalyst and maximizing the interfaces betweencatalyst, gas and electrolyte at which the key electrochemical processesoccur. The slurry made from GEFC-IES or Nafion® is suitable for use inhalftone screen printing.

As illustrated in FIG. 1, an exemplary oxygen sensor 10 can be carriedin a housing 12 and include, a gas diffusion sensing or workingelectrode 14, a reference electrode 16 and a counter electrode 18. Oneor more of the electrodes can be formed by a printing process asdescribed below in detail. The electrodes need not be identical.

As would be understood by those of skill in the art, electrodes formedby the present printing based process can be incorporated into gasdetectors, such as detector 30. Detector 30 can include a housing 34which carries the sensor 10, as well as electrodes 14-18 manufactured asdescribed herein. Control circuits 36 can be coupled to the electrodesto make gas concentration determinations. An audio and/visual outputdevice 38 can be provided to alert users to a current, sensed gasconcentration.

FIG. 2 illustrates aspects of a method 100 in accordance herewith. Aslurry, including a platinum catalyst along with glycol and a solutionof GEFC is mixed together to get a uniform mixture as at 102. The slurryis then heated, to a certain volume, as at 104.

The screen printable catalyst is then halftone printed on a PTFE sheetusing a printer, as at 106. The printed element or shape is thensintered at a predetermined temperature, as at 108, to obtain anelectrode which can be used as a sensing, reference, or counterelectrode, as at 110.

In accordance herewith, the electrode catalysts can be made from 80%weight Platinum black and 20% weight of GEFC-IES binder. The binder inthe slurry not only affects gas diffusion parameters in the sensor italso supports the platinum electrocatalyst and maximizes the interfacesbetween catalyst, gas and electrolyte at which the key electrochemicalprocesses occur.

Relative to FIG. 1, sensor 10 can be implemented as a O₂ sensor or COsensor using the electrode created by the above described process 100.Operationally, at the sensing electrode for an O₂ sensor the O₂ isreduced:

O₂+4H⁺+4e ⁻→2H₂O  (1)

At the counter electrode there is a counter balancing oxidation:

2H₂O→4H⁺+O₂+4e ⁻  (2)

FIG. 3 illustrates a graph of the response of an O₂ sensor with time to20.9% O₂ in air and N₂ with respect to the above described catalystmaterial. At a sensing electrode for a CO sensor the CO is oxidized:

CO+H₂O→CO₂ 2H⁺+4e ⁻  (3)

At the counter electrode there is a counter balancing reduction:

O₂+4H⁺+4e ⁻→2H₂O  (4)

FIG. 4 illustrates a graph of the response with time to air and 50 ppmcarbon monoxide using a mixture of GEFC-IES and Platinum as a sensingelectrode formed by screen printing.

In another example a predetermined ratio of platinum and graphite ismixed together with glycol and a solution of GEFC-IES to get a uniformmixture. Then the slurry is heated to a predetermined volume. Thecatalyst is then halftone printed on a PTFE sheet using a printer. Afterprinting, the printed element is then sintered at a predeterminedtemperature to obtain the electrode which can be used as a sensing,reference, or counter electrode for an O₂ sensor.

The electrode catalyst in this second example is made from 75% weightPlatinum black, 10% weight graphite and 15% weight GEFC-IES binder. FIG.5 illustrates a graph of the response to O₂ sensor with time in air andN₂ with respect to the second catalyst material.

In summary, the above disclosed electrode manufacturing process usingscreen printing method has fewer steps than known processes. First, acatalyst (e.g. Platinum Black or mixture of Carbon and Platinum Black orother noble metal catalyst) is mixed with GEFC-IES or Nafion® or amixture of GEFC-IES and Nafion®. Glycol is then added to form a slurryby stirring.

An electrode form can then be screen printed on a PTFE membrane andsintered at an elevated temperature. Platinum electrodes usable in bothsensors and CO sensors can be formed using this screen printing process.

Those of skill will also understand that the graphs of FIG. 3-5 areillustrative only and not limitations hereof. Variations in electrodestructures may lead to differing response times without departing fromthe spirit and scope hereof.

From the foregoing, it will be observed that numerous variations andmodifications may be effected without departing from the spirit andscope of the invention. It is to be understood that no limitation withrespect to the specific apparatus illustrated herein is intended orshould be inferred. It is, of course, intended to cover by the appendedclaims all such modifications as fall within the scope of the claims.

Further, logic flows depicted in the figures do not require theparticular order shown, or sequential order, to achieve desirableresults. Other steps may be provided, or steps may be eliminated, fromthe described flows, and other components may be add to, or removed fromthe described embodiments.

1. An apparatus comprising: a housing; and an electrochemical gas sensorcarried by the housing, wherein the electrochemical gas sensor comprisesa plurality of electrodes, and wherein at least one of the plurality ofelectrodes comprises a printed portions, wherein the printed portioncomprises a slurry printed on a substrate.
 2. An apparatus as in claim1, wherein the substrate is a porous membrane.
 3. An apparatus as inclaim 1, wherein the slurry comprises a selected catalyst.
 4. Anapparatus as in claim 3, wherein the slurry comprises at least aselected binder.
 5. An apparatus as in claim 1, wherein the slurry issintered to the substrate.
 6. An apparatus as in claim 1, wherein theslurry comprises at least a platinum catalyst and a binder. 7.(canceled)
 8. An apparatus as in claim 1, wherein the slurry is halftoneprinted on the substrate.
 9. An apparatus as in claim 1, wherein theelectrochemical gas sensor comprises an electrolyte in contact with theat least one of the plurality of electrodes, wherein the binder, atleast in part, provides an interface between a catalyst in the slurry, agas in contact with the substrate in the sensor, and the electrolyte.10. An apparatus as in claim 9, further comprising: a second housing,wherein the second housing carries the housing, the sensor, and controlcircuits coupled to the electrodes to establish an ambient gasconcentration.
 11. An apparatus as in claim 10, further comprising anaudible or visual output device, wherein the audible or visual outputdevice is carried by the second housing and coupled to the controlcircuits to provide a gas concentration indicator.
 12. A gas detectorcomprising: an electrochemical gas sensor, wherein the electrochemicalgas sensor comprises a plurality of electrodes carried spaced apart fromone another, at least one of the electrodes is formed of acatalyst/binder slurry which is halftone printed onto a substrate, andwherein the catalyst/binder slurry and the substrate are sinteredtogether to form the electrode.
 13. A gas detector as in claim 12,wherein the slurry comprises at least a noble metal catalyst and aselected binder.
 14. A gas detector as in claim 13, wherein thesubstrate comprises a selected porous membrane.
 15. A gas detector as inclaim 14, further comprising control circuitry coupled to the gassensor, wherein the electrochemical gas sensor is configured toestablish an ambient gas concentration of at least one gas.
 16. A methodcomprising: providing a slurry comprising at least a catalyst and abinder; printing the slurry in a selected shape onto a porous substrate;and sintering the printed shape and associated substrate.
 17. A methodas in claim 16, further comprising: incorporating the sintered shape andthe associated substrate as an electrode into an electrochemical sensor.18. A method as in claim 16, wherein the slurry further comprises adiluent.
 19. A method as in claim 16, further comprising: heating theslurry prior to printing the slurry.
 20. A method as in claim 16,wherein the catalyst comprises at least one of: platinum, platinumblack, a mixture of graphite and platinum, a mixture of carbon andplatinum black, a noble metal, or mixtures thereof.
 21. A method as inclaim 16, wherein the binder comprises at least one of a perfluorinatedion electrolyte solution or a copolymer of tetrafluoroethylene and asulfonic acid.